The present invention generally relates to liquid crystal displays(LCDS), more particularly to an optically compensated splay mode LCD having enhanced picture quality.
Nevertheless its narrow viewing angle characteristic, the twisted nematic(TN) mode LCDs have been widely used for notebook computers. However, in order to substitute the cathode ray tube(CRT) in the monitor and television set market, it is a preconsideration of LCDs to have wide viewing angle characteristic.
As a recent development to improve the TN mode LCD""s viewing angle characteristic, a technique to form a dual domain in a liquid crystal layer of a TN-LCD and an IPS-LCD have been suggested. Herein, followings are the technique to form the dual domain in the liquid crystal layer; (1) a multiple rubbing method; (2) a multiple alignment layer method; (3) an edge fringe field method; and (4) parallel fringe field method.
However, those methods require cumbersome manufacturing steps. For example, in the case of the multiple rubbing method, it requires more than one rubbing step and photolithography step for each panel or two substrates. In the case of the multiple alignment layer method, alignment layer patterning and etching steps for one substrate or two substrates are required. In the case of the parallel fringe field method, a patterning step for an ITO layer on a color filter is required. Those steps in the foregoing three methods include the further steps of coating, baking, patterning, developing and removing photoresist. Furthermore, the multiple rubbing method requires a rubbing step for an additional layer, the multiple alignment layer method requires a step of coating an additional layer, or the parallel fringe field method requires an etching method at the color filter portion. Therefore, the manufacturing process of the dual domain is more complicated than that of the conventional single domain, also costs a greater deal. Moreover, viewing angles in the multiple rubbing method are asymmetric.
Further, the IPS-LCD has great performance in the viewing angle characteristics, however its transmittance and aperture ratio are very low and the response time is very slow.
As a result, there has been suggested an optically compensated bend (OCB) mode LCDs compensating the birefringence of liquid crystal molecules so as to obtain the uniform viewing angle characteristic at all directions without requiring numbers of rubbing steps, as well as to the response time characteristic. (See reference: SID 93 Digest page. 277, xe2x80x9cWide-Viewing-Angle Display mode for the Active-Matrix LCD Using Bend-Alignment Liquid Crystal Cell, Y. Yamaguchi, T. Miyashita, T. Uchida).
FIGS. 1A to 1C are cross-sectional views for illustrating the constructions and the operation of a conventional OCB mode LCD.
As shown in FIG. 1A, a lower substrate 10 and an upper substrate 15 are opposed with intervening a selected distance therebetween. A liquid crystal layer 18 having a plurality of liquid crystal molecules 18a is sandwiched between the lower and the upper substrates 10,15. Herein, the liquid crystal molecules 18a have, for example, the positive dielectric anisotropy. At inner surfaces of the lower and the upper substrates 10,15, a pixel electrode 11 and a counter electrode 16 for driving the liquid crystal molecules 18a are formed respectively. Further, a first alignment layer 12 is disposed between the liquid crystal layer 18 and the lower substrate 10 including the pixel electrode 11, a second alignment layer 17 is disposed between the liquid crystal layer 18 and the upper substrate 15 including the counter electrode 16. The first and the second alignment layers 12,17 are homogeneous alignment layers having the pre-tilt angle of below 10xc2x0 C., and they are rubbed in a direction parallel to each other. First and second polarizing plates 19a,19b are disposed at outer surfaces of the lower substrate 10 and the upper substrate 15 respectively. Herein, the polarizing axes of the first and the second polarizing plates 19a,19b are arranged to cross each other, and one of the polarizing axes forms a selected angle with respect to the rubbing direction, for example approximately 45xc2x0 C. or 135xc2x0 C.
Operation of the OCB mode LCD is as follows.
First of all, as shown in FIG. 1A, the liquid crystal molecules 18a are arranged in a splay type according to the influence of the first and the second alignment layers 12,17 when no voltage difference is occurred between the pixel electrode 11 and the counter electrode 16. As a result, the polarizing state of an incident light passing through the liquid crystal layer is changed, and the light is leaked while passing the second polarizing plate 19b. 
Meanwhile, when a voltage above the critical voltage Vs is applied to the pixel electrode, the liquid crystal molecules 18a in a middle layer as shown in FIGS. 1B and 1C are arranged such that their long axes are almost parallel to an electric field. As a result, the liquid crystal molecule arrangement of the splay type is changed into a bend type. Herein, FIG. 1B illustrates the liquid crystal molecule arrangement when the critical voltage Vs is applied to the pixel electrode, and FIG. 1C illustrates the liquid crystal molecule arrangement when the voltage above the critical voltage Vs is applied to the pixel electrode.
When voltage is applied, the splay state of the liquid crystal molecule arrangement is changed into the bend state that long axes thereof are parallel to the electric field because an electric field force directly loaded on the middle layer are not bearable.
Furthermore, when the voltage above the critical voltage is applied to the pixel electrode, liquid crystal molecules 18a of not only in the middle layer but also around the alignment layers are arranged their long axes to be parallel to the electric field thereby reducing the amount of transmitted light and also becoming gradually dark state. At this time, the OCB mode LCD employs the maximum amount of light as the white state during the bend state, and the OCB mode LCD employs as the dark state the minimum amount of light when the voltage above the critical voltage is applied to the pixel electrode. That is, the voltage below the critical voltage is not used for the display mode.
Since the liquid crystal molecules 18a are arranged to form symmetries up and down with respect to the middle layer when the electric field is formed, the phase of the liquid crystal molecules is naturally compensated while light is passing through the lower substrate 10 to the upper substrate 15. Furthermore, since the dark state is obtained when a predetermined critical voltage is applied to, the backflow is not occurred and relatively fast response time is obtained.
However, the OCB mode LCD has the following problems.
General LCDs include spherical or elliptic spacers(not shown) dispersed in the liquid crystal layer so as to maintain the cell gap. At this time, the liquid crystal 30 molecules in the conventional OCB mode LCDs are in the splay state and arranged along curved faces of the spaces before the electric field is applied. That is to say, as shown in FIG. 2, as the liquid crystal molecules 18a are arranged along the curved faces of the spacers in the splay state, the liquid crystal molecules in the spacer-disposed portions are arranged insecurely. Accordingly, the initial liquid crystal molecule arrangement is insecure, change of the splay state into the bend state is insecure when a high voltage above the critical voltage.
Furthermore, as shown in FIG. 3, the pixel electrode 11 is formed as a pattern and having a predetermined step difference, the first alignment layer 12 is formed on the pixel electrode 11. As a result, the long axes of the liquid crystal molecules 18a on the first driving electrode 11 are parallel to the first alignment layer 12. However, the liquid crystal molecules 18a at the step difference portion of the pixel electrode 11 are arranged such that their long axes are parallel to the stepped face, thereby occurring the reverse tilt.
As the alignment of the liquid crystal molecules is partially bad and the reverse tilt is occurred step difference portion, picture quality of device is degraded and it is very difficult to change uniformly its splay state to the bend state even though a high voltage above the critical voltage is applied thereto.
Moreover, although the critical voltage is applied to the pixel electrode, it takes a considerable amount time to change the splay state into the bend state. Also the change is not uniform per pixel. As a result, the high voltage above the critical voltage should be applied to change quickly the splay state into the bend state with uniformity.
Accordingly, it is the object of the present invention to provide an LCD capable of obtaining wide viewing angle as well as improving picture quality.
To accomplish the foregoing object of the present invention, an LCD comprises: lower and upper substrates opposed with intervening a selected distance and having driving electrodes in their inner surfaces respectively and a liquid crystal layer sandwiched between the lower and the upper substrates and having a plurality of liquid crystal molecules, wherein no voltage is applied to the driving electrodes, the liquid crystal molecules are arranged in a bend state, and wherein a voltage above a critical voltage is applied to the driving electrodes, the liquid crystal molecules are arranged in a splay state which makes a symmetry of up and down with respect to a middle layer of the liquid crystal layer.
Further, the LCD comprises: lower and upper substrates opposed with intervening a selected distance and having driving electrodes in their inner surfaces respectively; a liquid crystal layer sandwiched between the lower and the upper substrates and having a plurality of liquid crystal molecules; first and second alignment layers disposed at the respective inner surfaces of the lower and the upper substrates; and first and second polarizing plates disposed at the respective outer surfaces of the lower and the upper substrates, wherein the liquid crystal molecules have negative dielectric anisotropy, wherein the first and the second alignment layers have pre-tilt angle of below 90xc2x0 and they are rubbed in a direction parallel to each other, wherein one of polarizing axes of the first and the second polarizing plates forms a selected degree of angle with a rubbing axis.
The LCD of the present invention still comprises: lower and upper substrates opposed with intervening a selected distance and having driving electrodes in their inner surfaces respectively; a liquid crystal layer sandwiched between the lower and the upper substrates and having a plurality of liquid crystal molecules; first and second alignment layers disposed at the respective inner surfaces of the lower and the upper substrates; first and second polarizing plates disposed at the respective outer surfaces of the lower and the upper substrates; and a phase compensation plate between the upper substrate and the second polarizing plate, or between the lower substrate and the first polarizing plate, wherein the liquid crystal molecules have negative dielectric anisotropy, wherein polarizing axes of the polarizing plates are perpendicular to each other, wherein the first and the second alignment layers have pre-tilt angle of below 90xc2x0 and they are rubbed in a direction parallel to each other, wherein one of the polarizing axes of the first and the second polarizing plates forms a selected degree of angle with a rubbing axis.